1. Field of the Invention
The present invention relates generally to laminated parts for use in electric motors and generators. More particularly, the present invention relates to electric motor or generator stators and rotors having stacked laminas and the methods and machines, i.e., progressive dies and controllers therefor, used in the manufacture of such laminated parts.
2. Description of the Related Art
The manufacture of stators and rotors employing stacked laminas is well known in the art. Typically, the laminas are blanked from a continuous strip stock and then stacked and bound together to form the stator or rotor. Progressive die assemblies for producing stator or rotor laminations wherein a strip of lamination material is fed through a sequence of punching steps to progressively form the laminas are also well known.
Rotor laminas generally include a plurality of skewed conductor slots which are formed around the periphery of the rotor stack in arcuately spaced relation to one another by rotationally indexing the laminas with respect to the rotor stack axis. Indexing involves rotating the rotor stack and the last produced lamina relative to each other by a predetermined rotational increment so that, when the laminas are combined in a stack, the rotor conductor bar slots defined by adjacent conductor slots are skewed or slanted relative to the stack axis. Stator stacks, on the other hand, include winding slots around the inner periphery of the stack which extend parallel to the stack axis, without skew, and are shaped to receive the stator windings. Although relatively rare, inside-out motors are often used in fan motor designs and utilize inner cores manufactured with straight slots to be used with outer cores having skewed slots, in which case it would be the outer core laminations which are rotated relative to each other to produce the skew angle.
In addition to producing a skew angle, as discussed above, the relative rotation of a lamina and the lamina stack onto which the lamina will be placed can be used to overcome the negative effects created by thickness inconsistencies in the strip stock from which the laminas are blanked. The strip stock used to manufacture lamina stacks may have thickness inconsistencies wherein one side of the strip stock is thicker than the other side. If laminas punched from the strip stock are stacked without rotation, the lamination stack produced will not have a uniform height around its entire perimeter because the stacking of the individual laminas will additively compound the thickness inconsistencies present in the strip stock on one side of the stack. A stack having a substantially uniform height can be produced, however, by rotating individual laminas relative to the lamina stack to equally distribute the thickness inconsistencies about the stack axis. The inconsistencies thereby cancel each other out in a known process often referred to as gamma correction.
In response to these problems, an autorotation system to compensate for nonuniform stock thickness was developed which both rotates and interlocks the stacked laminas. This system compensates for variations in lamina thickness while properly skewing the conductor slots of rotor laminas, as described in U.S. Pat. Nos. 4,619,028; 4,738,020; 5,087,849 and 5,123,155, all assigned to the assignee of the present invention and the disclosures of which are incorporated herein by reference. In the systems disclosed in the aforementioned patents, the choke barrel holding the lamination stack is automatically rotated before each lamina is blanked from the strip stock and the lamina's circumferentially disposed tabs are interlocked with the slots of the uppermost lamina of the incomplete lamination stack within the barrel.
In the apparatus and method disclosed in the aforementioned patents, the individual laminas are typically rotated through an angle which is the sum of 180.degree. to provide for gamma correction plus a smaller incremental amount necessary to provide the proper skew angle. Although the laminas may be rotated through other angles, the angle, without including the incremental rotation necessary to produce the skew angle, must be at least 360.degree./(number of interlock tabs) to permit the use of interlocking tabs and slots.
The use of an AC or DC servo motor and a controller allows the operator of the die assembly to rapidly and easily alter the rotational amounts to produce different skew angles or to alter the angle through which the laminas are rotated to correct for thickness inconsistencies. The controller computes a single angle through which the choke barrel must be rotated to provide for both gamma correction and a proper skew angle. The use of a servo motor to rotate a choke barrel for such purposes, however, places a limit on the number of strokes per minute of the die assembly because it requires the use of a relatively large, and relatively slow, servo drive unit which has the capacity to handle the inertial loads involved in rotating the choke barrel through such large angles between each stroke of the die assembly as well as to be sufficiently accurate. Typical achievable rates are 275 to 300 strokes/minute.
Mechanical indexers which utilize a camming assembly such as a roller cam to provide indexed rotational movements to rotate the choke barrel are also known and may be used to provide for gamma correction while allowing the die assembly to operate at significantly higher rates, e.g., in the range of 400 to 500 strokes/minute, than a comparable servo drive assembly. Mechanical indexers used for gamma correction lack the flexibility of servo drive systems, however, since the angle at which they rotate the choke barrel cannot easily be adjusted.
A mechanical system has also been developed to provide for the incremental indexing of laminas to provide a proper skew angle and U.S. Pat. No. 3,203,077 provides one example. Although such a mechanical system provides for some adjustment of the amount of rotational indexing necessary for a skew angle, the adjustment of mechanical indexing systems is not as flexible or convenient as the adjustment of a servo drive motor which is regulated by a controller.
A system which utilizes a modified roller cam assembly to provide a mechanical indexer in conjunction with a system for providing an incremental rotational movement for producing a skew angle is disclosed by U.S. Pat. No. 5,163,217. The disclosed system, however, cannot employ a standard "off the shelf" roller cam assembly without substantially modifying the assembly thereby increasing the cost of the system. The disclosed system also employs a plurality of small rollers which are in frictional contact between an input roller and an output disk. The use of such frictionally engaged surfaces to transmit rotational movements is more subject to slippage than an arrangement involving the transfer of rotational movement by positively locking gear teeth.
What is needed is a system for relatively rotating a lamina and a stack which overcomes the limitations of the prior art.